There is a general need for more efficient practices for the chemical reduction of rare earth element compounds to the respective rare earth metals. This need is especially prevalent with respect to the rare earth elements neodymium and praseodymium because of their increased utilization in iron-neodymium (and/or praseodymium)-boron type permanent magnets. As the many different types and forms of these magnets compete for applications, it is desirable to produce the rare earth constituent(s), neodymium and/or praseodymium, at lower cost.
My earlier patents describe the metallothermic reduction of rare earth oxides dispersed in a molten salt reaction medium (typically comprising calcium chloride) using elemental calcium as the reductant (U.S. Pat. No. 4,578,242) and the reduction of rare earth chlorides in the same type of reaction medium (U.S. Pat. No. 4,680,055). The metallothermic reduction of rare earth oxides such as neodymium (and/or praseodymium) oxide has been used commercially to produce neodymium metal for the types of permanent magnets described above. The use of the molten salt reaction medium in combination with a molten neodymium-iron alloy product recovery phase produces high purity neodymium metal in good yield, about 95 percent yield. However, a mixture of byproduct calcium oxide and calcium chloride is formed, which requires a disposal decision. Furthermore, the required volume of calcium chloride reaction medium with respect to the yield of neodymium metal is fairly high, which reduces the capacity and the efficiency of the reactor. Similar drawbacks are associated with the use of neodymium chloride as a starting material. Furthermore, the use of neodymium chloride requires that the most readily available form of this material must be dried and made anhydrous before it can be used in the very hot molten salt bath.
Other workers, Seon et al, U.S. Pat. No. 4,636,353, have described a reduction process for rare earth fluorides like neodymium fluoride that utilizes a mixture of calcium chloride, calcium and iron. These materials are simply mixed together in finely divided particle form and heated without stirring to a high temperature, preferably 900.degree. C. to 1100.degree. C. A reaction product mass is formed that comprises a calcium chloride-calcium fluoride slag, residual calcium and a mixture of iron and neodymium. The yield in this practice is relatively low, the reaction temperatures are high, and separation of the slag from the metallic product is not easy and convenient.
It is an object of this invention to provide an improved method for producing rare earth metals from their corresponding fluorides in high yield and high quality. It is a further object that such process be volumetrically efficient in that it produces a high yield of rare earth metal per unit volume of reactor vessel space.
It is a further object of this invention to provide such a process that is particularly useful for producing neodymium and/or praseodymium metal alloys using a stirred molten calcium chloride-based reaction medium and calcium metal as the reductant.
It is a further object of this invention to provide such a reaction scheme whereby calcium fluoride byproduct is recycled as a reactant for forming neodymium fluoride from another neodymium (i.e., rare earth) starting material.
In accordance with a preferred embodiment of my invention, these and other objects are accomplished as follows.